Thyroid transcription factor 1 (TTF-1) is a 38kDa homeodomain-containing nuclear transcription factor belonging to the NKX2 gene family. TTF-1 expression was found in early stages of gestation and could play an important role in cell differentiation and morphogenesis of the thyroid and lung. TTF-1 is expressed in epithelial cells of the lung and thyroid in normal tissues. TTF-1 is also used as a marker for distinguishing lung and thyroid carcinomas. Additional studies showed TTF-1 is expressed in endometrial carcinoma and small cell carcinomas of the prostate and bladder. A panel of antibodies would be valuable in identifying tumors of lung or thyroid origin.